Subject: [Isis-wg] SPF optimization

From: Chaitanya Huilgol <[email-masked]>
Date: Sat, 26 Jan 2002 14:19:37 +0000

hi all
  I have a Question regarding addition of IP reachability entries 
  in to the forwarding database, without recomputation of SPF.

ref rfc1195 AnnexC: 
 
3) If only End System LSP information has changed,it is not
 necessary to recompute the entire Dijkstra tree. If the proper 
data structures are used, End Systems (including IP reachability
entries) may be attached and detached as leaves of the tree and 
their forwarding information base entries altered as appropriate.

-- I believe IF the IP reachibality entries are taken from a static
Table
   there is very little chance of such a change in info.
   
now my question is

(1)If an IS advertise a new IP reachability, How can you be
sure that the shortest PATH to this network is through this IS.
it is quite possible that a route already exists with a
smaller/larger 
cost through another IS in the forwarding database.
(2)Again if you want to remove an entry how can be sure that the path

was calculated through the IS.
  for Eg:

     10     10
 R1------R2------R3
 |               |--IP/24
 |     10        | 
 |---------------R4  

  if R1 runs SPF.route to IP/24 through R4
  later if R3 reports IP/24..How do u Know that the entry 
  in forwarding database was through R4 which is the shortest 
  and you should not replace.

--In case R3 and R4 have advertised IP/24
  and R1 calculates path through R4 
  and then R3 dose not advertise IP/24
  --you should not be removing the entry for the forwarding DB 

 ---finally do the implementations support this optimization.

---------------------------------------------
Chaitanya Huilgol.
[email-masked]
B.M.S college of Engineering.
Bangalore,India.
---------------------------------------------

_______________________________________________
Isis-wg mailing list  -  [email-masked]
http://external.juniper.net/mailman/listinfo/isis-wg